Method and apparatus for frequency response measurement



March 11, 1952 R. JOHNSON 2,588,730

W. METHOD AND APPARATUS FOR FREQUENCY RESPONSE MEASUREMENT Filed May 2, 1947 2 SHEETS-SHEET l Stwentor .WAYNE/6 dbH/vsaN;

Cttorneg March l1, 1952 w. R. JOHNSON 2,588,730

METHOD AND APPARATUS EUR FREQUENCY REsPoNsE MEASUREMENT 2 SHEETS-SHEET' 2 Filed May 2. 1947 UR @NNN .h @NRT www Patented Mar. 1l, 1952 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR FREQUENCY RESPONSE MEASUREMENT Wayne R. Johnson, Los Angeles, Calif.

Application May 2, 1947, Serial No. 745,480

6 Claims.

My,v invention relates generally to equipment for electrically detecting, amplifying, transmitting,l receiving, and reproducing sound and like signals of near-audio frequency. More particularly, my invention relates to a method and apparatus for measuring and/or recording the frequency response of equipment of the class described. A customary method used in laboratories and by service technicians for obtaining quantitative measurements of frequency response in a particular piece of apparatus, is to feed into such output signal level of the apparatus under test is measured for each input signal and the result presented in graphic or tabular form. When such information is presented in graphic form it is generally referred to as the frequency response curve for the particular apparatus tested.

Such procedure has a number of disadvantages, the most obvious of which is that it is tedious and time-consuming. For example, to obtain an adequate number of points from which to plot a response curvefrcm an audio amplifier over the normally audible range, requires on the order of twenty separate readings, and ifl extreme accuracy is required, each reading should be repeated several times.

Another disadvantage following as a corollary tothat just mentioned is the fact that in constructing or repairing audio or similar equipment, it is often desired to observe the effect of achange of ,one vof the electronic components of the equipment on the overallfrequency response. An accurate overall picture of the frequency response requires that the aforesaid step-by-.step procedurebe followed. For example, a change in the value of a resistor in an amplifier may correct a deficiency of output in the relatively high frequency range, while at the same timeintroducing an undesirable response peak in the lower frequencies.' This' undesirable peaking inv the lower frequencies maybe obvious ,from listening tothe output of the amplifier, 'but to obtain a quantitative appraisal, it is necessary to lplot or tabulate a new set cf output values. Thus, since it isfrequently necessary to plot a newresponse curve each time a slight Achange is made in the value of some component, the design and development of laudio equipment is a rsomewhat lengthy procedure if conventional testing methods are employed. E Y z Still anotherdisadvantage in the step-by-step method lis that, if Ian? accurateand 'stable result is to be obtained, a relatively expensive signal generator is required; for example, a beat frequency oscillator. This expense can, in some instances, be avoided by the use of standard Afrequency sound records. Such records, however, have prior to my invention been available only in the forms of either Aa single continuously varying note, or as a seriesof step-by-step discrete frequency signals.

" As used herein the term fnote designates a single integral audio signal which may be either of a constant frequency throughout its duration or continuously changing in frequency.

One method of frequency response testing which has had limited success in overcoming some of the above difficulties involves the use of a sweep frequency signal generator. Such a generator produces a series of signals, each consisting of a nelatively rapidly rising note, such signals being repeated at a rate whereby they may be reproduced visually on an oscilloscope screen'to produce a standing pattern. Each of the fsweep frequency signals includes all the frequencies over the range which it is desired to test'l and the amplitude of all frequencies is either the same throughout the pattern or is changed at a known rate throughout the pattern.

Thus'if the signals of such a sweep frequency generator are fed into equipment under test. and the output of the tested equipment is in turn fed into an oscilloscope, the amplitude modulation envelope ofthe pattern appearing on the oscilloscope screen will'be, in fact, a frequency respon'se curve for the equipment being tested.

Without more, however, the sweep frequency generatorl method produces a largely qualitative result. From the observation of a modulation envelope having no reference points thereon, only a general picture of the overall frequency response can be obtained. Such a pattern, for example, doesv not show the specific location Within the tested range" of peaks, dips, or other anomalies in the frequency response.

It is possible,-by the raddition of a relatively complex timing circuit, to mix with the sweep frequency signals above referred to, a signal comprising a series of marker pulses which appear on the oscilloscope screen and can be used to identify the frequency at Various points along the wave pattern. The technique for producing such marker pulses is well known in the art, and is used, for example, in producing time or distance markers on radar screens.

Testingvapparatus as just described which includes not only a sw eep frequency generatonbut a circuit for producing marker pulses, may be used experimentally in a testing laboratory but is far too cumbersome to be used in routine audio testing and is far too costly to be employed by the ordinary technician or experimenter. Furthermore, because of the nicety of adjustment required to maintain the marker pulses in the proper position on the signal wave pattern, it is likely that much more time would be spent in the laboratory adjusting the equipment that` would be saved by avoiding the use of the stepby-step test method. Still further, due to the unavoidable instabilities in electronic components such as resistors, condensers, chokes, vacuum tubes, etc., the composite equipment, once adjusted, would remain in such properly adjusted condition for only a relatively short time, after which readjustment would be necessary.

Bearing in mind the desiderata and difficulties just discussed, it is a major object of my invention to provide a method and apparatus for frequency response measurements which yield quantitative as well as qualitative results.

Another object of my invention is to provide apparatus for conducting tests of the class described which is relatively stable and requires no periodic readjustment.

Still another object of my invention is to provide test equipment as described which is readily portable and within the economic means of ordinary experimenters.

A further object of my invention is to provide testing apparatus which is particularly suitable for testing phonographic reproducers or tone heads.

The foregoing and other objects and advantages of my invention will become apparent from a consideration of the following description of a method and apparatus embodying my invention.

Briefly, my novel method includ"s the steps of Producing a series of rising notes or sweep frequencysignals with an adjustable frequency oscillator or similar signal generator, such note including the range of frequencies over which it is desired to obtain response measurements;

Controlling the operation of said generator with a modified sawtooth generator so that the rate of frequency increase during one note or signal is logarithmic and so that the repetition rate of the signals is on the order of the sweep rate for visual Oscilloscopes;

Producing in a pulse generating circuit a trigger pulse, and a series of marking pulses spaced at predetermined time intervals from the trigger pulse;

Mixing the sweep frequency signals and the pulse signals in such phase relationship that each trigger pulse coincides with the beginning of the sweep frequency signal and the marker pulses fall at various predetermined points along the sweep frequency pattern so as to identify various reference frequencies therein; and

Recording the combined output of the mixed signals on a permanent sound record.

The permanent record thus produced may be played through apparatus to be tested and the output of such apparatus fed into a cathode ray oscilloscope. The trigger pulse above referred to serves to trigger the sweep circuit of the oscilloscope so that the successive sweep frequency signals are repeated in the same position on the screen.

It is possible to reproduce the permanent sound record in a wide variety of different sound recording media; for example, magneticallyin La steel wire or tape, magnetically in a paper tape coated with ferromagnetic material, photograph- `ically in a conventional variable area or a variable density film sound track by engraving in a plastic material, etc. Without meaning to be limited to the particular sound recording method described herein, I prefer at present to record the output of the combined sweep frequency and pulse circuits in a conventional disc phonograph disc record from which duplicate pressings can be made. Thus I provide a relatively inexpensive but accurate piece of test equipment, the cost of which is well within the means of ordinary technicians and experimenters.

Such a pressing may be played with a relatively high quality tone head through equipment under test to obtain an accurate. overall picture of the frequency response without the necessity of reading and plotting a large number of separate measurements.

A record of the type just described has the additional advantage that it can be used to test tone heads themselves, the tone heads under test being used to play the record and the output thereof fed directly into a cathode ray or other type `of oscilloscope. y

Still another advantage which I have achieved. is that by the use of the record just described the transient response of a piece of audio equipment can readily and quickly be observed. Since thev marker pulses used are of extremely high frequency, they can be regarded as the equivalent of a square wave insofar as the transient oscillations produced thereby are concerned. Thus by greatly expanding the sweep rate of the oscilloscope and observing the wave pattern immediately following one of the marker pulses, the transient response of the equipment under test can be seen.

For a more detailed descriptionof the method and apparatus embodying my invention, reference should now be had to the accompanying drawings illustrating a preferred embodiment thereof, in which:

Figure 1 isa graphic representation of. a logarithmically increasing sweep frequency signal;

Figure 2 is a graphic representation of the signal shownin Figure 1 with a trigger pulse and a series of marker pulse signals superimposed thereon; p

Figure 3 is a block diagram illustrating the apparatus and connections employed to produce the signal illustrated in Figure 2;

VFigure 4 is a block diagram illustrating the use of .a sound record produced in accordance with my invention being used to test a piece of audio equipment;

Figure 5 is a circuit diagram of a novel sweep frequency control circuit combined with a pulse generator circuit according to my invention as employed in the arrangement illustrated in AFig" ure 3; and,

Figures 6 through 10 illustrate a number of oscilloscope patterns such as are obtained when using a sound record of my invention to test audio equipment.

Referring now to the drawings, particularly to Figures 1 and 2 thereof, it will be noted that the low frequencies, i. e., those to the left in Figure l, are increasing at a relatively slow rate, while the high frequencies to the right in Figure 1 are increasing at a higher rate as the pattern proceeds from left to right, This is accomplished by frequency modulating the output of a conventional beat frequency oscillator I4 over therange of frequencies to be tested (in the present embodiment,v approximately ten kilocycles). In order to avoid crowding of the low frequencies, the modulation signal is, as above stated, logarithmic as indicated by the numeral in Figure 3, which. identifies a graphic representation of the signal delivered by a sweep control generator ltothe beat frequency oscillator |4. Thus the signal Adelivered by the beat frequency oscillator I4 consists of'a series of repeated rising notes indicated graphically .and identified by the numeral l1v in Figure T3. The notes |15 range from approximately' sixty cycles to approximately 10,000 cycles in frequency, and are repeated at a rate of approximately twenty times perV second, or .just above the visual persistence rate. This produces a standing and non-flickering pattern anddoes not require the use of persistent-screen oscilloscope tube. l

A s is we ll known in the art, beat frequency oscillators are inherently unstable, particularly in the'low-frequency range. In order to avoid this inherent instability, the sweep control generator |6 is` controlled by a small amount of locking voltage picked up from the output of the oscillator and delivered to the sweep control generator through conductor I3 in a manner to be described more detail hereinafter.

It will-be remembered thatone of the desir-v able features of a sweep frequency signal to be used`for testing frequencyresponse, is thatv the signal carried with it, both a triggering pulse for the purpose of synchronizing the oscilloscope, and a number of marker pulses for the purpose of identifying the various frequencies along the Wave pattern. In the present instance, such a triggering pulse is initiated by the sweep control generator l'and fed to a pulse generator 2| i which includes multi-vibrator circuits which act in responseto the trigger pulse 20 to produce a series of marker pulses such as indicated by the numeral'22. A signal comprising the trigger pulse andthe marker pulses is delivered to an amplifier 23`wherein this pulse signal is amplified and'inverted as indicated by the numeral 24.` The. amplified and inverted pulse signal 24 is fed into a bridge mixing pad |S wherein it is combined with the sweep frequency signal l1 to produce the composite final signal 25. As will b e noted .in Figure 43, the composite signal which ultimately is recorded and used to test frequency response, V comprises a; relatively high amplitude trigger pulse 26 and a series of marker pulses 2 1gsuperimposed on the sweep frequency signal l1. This composite signal 25 is adjusted in level by a gain set 28 of4 conventional designl from which it is delivered to a conventional sound recording apparatus (not shown).

The pulses 21 may be placed at any desired point along the wavepattern of the rising signal |1. In the present-embodiment, the pulses are placed at positions along the pattern, indicating the frequencies of 1, 3, 5, '1, and lokilocycles respectively, reading from left to right in the drawings. As wil be noted from an examination of Figure 2, it is desirable that the marker pulses,

particularly those in the lower frequency rangel fall on the crest of a signal Wave. To this end the multi-vibrator circuits in the pulse gener` ator-2| contain adjustable time constant determinant components. This arrangement will be decibel in meedeelthereinafter.f I

Turning now toka more detailed discussion of,

the` portion .offthe circuit which vincludes the 6;. sweep control generator I6 and the pulse generator 2|, reference should be had to Figure 5.

As previously mentioned, a portion of the output of the beat frequency oscillator is used to trigger or lock-in the sweep control generator I6 so that the resultant pattern appearing on the oscilloscope screen will remain stationary. The

connection from the beat frequency oscillator isaccomplished through the conductor I9 which leads'to a lter 30 which is adapted to pass fre- 34 of the thyratron type. As is customary, the,

thyratron `34 is provided with a cathode 35 and a plateor anode 3 6 in addition to the grid or control element 31. The cathode 35 is connected through a conventional resistor and capacitor connection to ground, whilev the plate 36 is connected through a fixed resistor 40, a fixed resistor 4|, and a variable resistor 42 to a source of positive voltage which is applied at the terminal43. i

AV capacitor 44 `of relatively large capacity has one of its terminals connected to the junction point 45 of resistors 40 and 4|, while the other terminal of the capacitor is connectedto the cathode 35. A resistor45 is connected between the Ipower supply terminal 43 and the cathode 35 to maintain the latter at a predetermined positive potential with respect to ground. The various capacitors and resistors have values selected so` that when a signal of a, proper frequency is ap-y sistor 48 is quite small, and the other terminal of r the capacitor is connected through resistors 4| and 42 to the positive power connection 43.

'When the thyratron is non-conductive, the capacitor 44 is charged through resistors 4| and 42 at a rate determined by the values of theseresistors. Since the plate 36 is not conducting any current under theseuconditi'ons, its potential is substantially the same as that of thepositive terminal of the capacitor 34, and will increase with it. v

' It will be understood, of course, that the presence of resistors 4|' and 42 causes the voltage at" junction 45 to increase exponentially, approaching the voltage of the power supply terminal 43 asymptotically as the capacitor 44 approaches its fully charged condition. However, when the grid 31 is triggered by the input signal, the thyratron 34 becomes conductive andthev capacitor 44 is discharged through it, the speed of this discharge being limited only by the protective resistor' 40.I

During the conducting portion of the cycle, the grid 31 has `no control on the thyratron 34, but once the plate voltage drops below a certain minimum value, the tube ceases to conduct, and the grid 31 regains control. The cathode 35`is normally maintained at -a value slightly positive with respect to ground by the resistor 43, while the grid 31 remains substantially at ground potential. or negativewith respect to the cathode until such time as an input signal is again passed throughv the .filter 3D.

The output of theuthyratron -34, as measured at. the junction 45, thus consists of an exponentially rising voltage. which suddenly drops sub.- stantially to zero or ground potential, and then repeats this cycle. By connecting a, capacitor 4T to the junction 45, these voltage variations may be transmitted to the pulse generator 2l, herein.- after described, as a single pulse occurring once each cycle. This, of. course, requires a. capacitor of small capacity in order that only the transient or high frequency components of the. signal be transmitted'.

If a. capacitor of greater capacity is used, the lower frequency components or general. wave shape of the voltage appearing atterminal will. be transmitted therethrough. I make use of this feature to. control the operation of a vari able impedance tube, which in turn controls the operation of the beat frequency oscillator I4. As indicated in Figure 5, I connect one terminal of a capacitor to the junction 45, the other ter` minal of the capacitor being connected through a resistor 5i to ground. The resistor 5i is of the type having a variable connection 52, and thelatter is connected through a resistor 53 tothe control grid 54 of an electron discharge tube 55. The tube 55 is one having a remote, cut-oir and is preferably a pentode of the type known as a 6AB'7/1853. It is to be understood that other suitable types may be used if desired, but I have found the tube mentioned to be very satisfactory. As indicated, this tube includes a cathode 55, a control grid 54, a screen grid 51, a suppressor grid 58, and a plate or anode 59. The cathode 56 is connected through a conventional self-biasing connection 60, consisting of a resistor and capacitor, to ground, while the suppressor grid 58 is connected directly to ground. A small` variable capacitor 6i is connected between the control grid 54 and the plate 55 to effect a 90Q phase shift between the plate and grid oi the tube, while a small fixed capacitor 52 connected between the wiper 52 and ground permits the passage of high frequency transients therethrough which might otherwise interfere with the proper operation oi the equipment.

In addition to being connected to the capacitor 6l, the plate 59 is connected through a, choke coil 53 to the terminal 43 connected to the power supply, while the screen grid 51 is connected through a resistor 64 to the same terminal of power supply; In addition, the screen grid 51 is connected through a capacitor 65 and a resistor 66 to ground so that alternating or transient currents which might otherwise appear at the screen grid will have no effect thereon. To complete the circuit; a small. capacitor El has one of its terminals connected to the plate 5B, while the other terminal is connected to an appropriate terminal on the beat frequency oscillator I4.

It will be apparent to those skilled in the art that the conductor 68 will have a reflected impedance appearing thereon which may vary from capacitive to inductive, depending upon the potential applied toA the control grid 54 of the electron discharge tube 55. This reilected impedance may be used to vary the frequency of oscillation of a circuit, and this is precisely what is done in the beat frequency oscillator I4. Such an oscillator, of which there are several well-known types available, includes a fixed oscillator and a variable oscillator whose outputs are combined or beat against each other to produce an audio frequency signal. By varying the impedance which controls the frequency of the variable Voscillator. the beat or audiofrequency output oi the device may be controlled. It may be shown that. by varying the potential of the grid 54. of the tube 55v from cut-off potential to saturation, the reected impedance appearing on the conductor 68 will vary logarithmically, and thus. I am ableto obtain. a reiiected. impedance. saw tooth signal. I5 whichv appears in the conductor`- 63 with the de# sired logarithmic decrement. In this wayr I am able to obtain a signal il of varying frequency as shown in Figure l, but to obtain the greatest advantagesV from my device, this. signal must be provided with a trigger. pulse and various marker pulses to indicate the. diierent frequencies. Con.- sequently, I have developed a pulse generator- 2| whose circuit is shown in Figure 5.

As previously mentioned, the capacitor 41- has one or its terminals connected to the junction 45 in the output circuit of the thyratron 34., and the other terminal of the capacitor is connected. by a conductor 'i0 to an input terminal 1I of. the pulse generator 2l. It will. beremembered that the signal appearing on the conductor 2t) is a single pulse of very short duration,` indicated` at 20 in Figure 3. This pulse 20 is the triggering; pulse. which is` used to start the sweep of. the viewing oscilloscope in a manner hereinafter described, and hence the magnitude or amplitude of this pulse should be quite large. In. addition, to indicate the variousv frequencies,

' marker pulses should be provided which have.- a.

smaller amplitude than that` of the triggering pulse 2.0, so that the sweep circuit of the oscilloscope will. not be operated by the marking pulses. Various methods may be used to genc crate the marker pulses, butsince the triggering.

pulse 20 always appears at the same portion of the cycle, I have found it convenient to use this puise to control the generation of the marker pulses.

As indicated in Figure 5, the, input terminal ll of the pulse generator 2| is connected. to a control element 'l2 of an electron discharge tube 13. the latter preferably being of the twin triode type such as a. 6C8. The first triode section includes a cathode 1d, the control element or grid 12, and a plate or anode '15, while the second section includes a cathode 16, a control element or grid l1, and a plato or anode 18. It is to be understood, of course, that while I have shown the electron. discharge tube 'I3 as consisting of a twin triode type, other types may be used and two separate electron discharge tubesV may bey used instead of the single tube if this should` be desirable.

The grid 'l2 of the first triode sectionv of the tube T3 is connected through a resistor 80 toground or the other input connection 8l, whileA the cathode 'i5 is connected directly to ground, thereby providing the desired bias for theA grid. The plate 15 is connected through a resistor 82 to a terminal 83 which in turn isY connected to the positiveterminal of a power supply (not shown) and the plate is also connected through a capacitor 84 to the grid 'lly of the second` triode section of the electron discharge tube 13; The cathodel i5 of this second triode section is connected through a resistor 85 to ground 8|, while the grid H is also connected through a much higher resistance 85 to ground. To complete the circuit, the plate 78 is connected throughs. resistor 81 to the positive terminal 83, to provide a circuit which is recognized as a standard two-stage resistance-capacitance coupled amplier.

The trigger pulse 25 is thus amplified in its passage through the electron discharge tube 13. and this amplified pulse is then used to control a multi-vibrator circuit of more or less conventional design. The multi-vibrator about to be described is adapted to provide a marker pulse at a predetermined point in the sweep of the frequency of the beat frequency oscillator I4, and a separate multi-vibrator is provided for cach of the other marker pulses. Since each of the 1` multi-vibrator circuits is identical to the others, only one of the circuits has been illustrated, the "one shown being adjusted to provide a marker pulse at 1,000 cycles per second, while the other circuits are adjusted to provide pulses at 3, 5,

and 7, kilocycles per second.

` In the particular multi-vibrator circuit I have chosen, the plate 18 of the second triode section of the electron discharge tube 13 is connected fto a junction point 90'and then through Va capacitor 9| to the grid or control element 92 oi 'an electron discharge tube 93, which, like the previously mentioned tube 13, may be of the twin -triode type such as a 6N7. As is customary in variable resistor to ground so that the grid will have the proper bias voltage impressed upon it.

As is customary in multi-vibrator circuits, the grid 92 of the rst triode section is connected through a capacitor |0| to the plate 98 of the second triode section; and this latter plate is also connected through a resistor |02 -to a tervminal |03, which'in turn is connected to the positive vterminal of a power supply (not shown). vThe plate 95 of the first triode section is connected through a resistor |04 to the positive teriminal |03, while the grid 91 of the second triode section is connected through a resistor |05 to the plate 95 of the rst triode section. To complete the circuit, the grid 91 of the second triode section is also connected through a resistor |06 to a terminal |01 which is connected to the negative terminal of a power supply (not shown). It will be understood that the power supply used in connection with this multi-vibrator circuit tmay be of any conventional type having a sufricient current capacity and provided with terminals which supply a voltage negative with respect to ground, a voltage positive with respect to ground, and a grounded or common return. :To providethe desired output voltage from the risulti-vibrator, the plate 95 of the rst triode section is connected through a capacitor |08 to -a junction point ||0 where a voltage output of the desired wave shape is secured.

In the operation of the multi-vibrator circuit just described, the amplied voltage of the trigger pulse 20 is applied to the grid 92 of the rst triode section so that the previously conductingV triode section is rendered non-conductive, while the previously non-conductive triode section is rendered conductive. After a predetermined interval'of time, the circuit automatically reverses the conductive and non-conductive triode sections thereby producing a substantially square, but non-'symmetrical voltage output wave form. A detailed analysis of the operation of such a multi-vibrator circuit may be found in many standard test books, and will not be repeated here, but it may be shown that =by varying the variable resistor |00, it is possible to control the relative time, between trigger pulses 26, when the conducting and non-conductingcharacteristics of the twin triode sections of the electron discharge tube 93 are reversed. The capacitor |08 acts, in elect, to change the square wave output of the multi-vibrator circuit to a series of oppositely directed pulses, the first pulse, corresponding to the trigger pulse 26, appearing at the junction point ||0 as a pulse of opposite polarity from the original trigger pulse The second or marker pulse, corresponding to the second reversal of the triode section conducting characteristics will appear as a pulse having the opposite polarity of the first pulse, and consequently having the same pulse as the original trigger pulse 26. The variable resistor |00 is adjusted so that the first marker pulse just described occurs at the 1,000 cycle per second point on the sweep frequency signal.

By connecting other multi-vibrator circuits to the junction points and H0, as indicated by the conductors ||1 and H8, respectively, marker pulses may be provided at 3, 5, and 7 kilocycle frequencies. Since each of these multi-vibrator circuits is identical with the one just described, it is felt that a detailed description of their construction and operation is not necessary. In each case, the variable resistor |00 is adjusted to provide a marker pulse at the desired point on the sweep frequency signal, and the accuracy of this adjustment may be checked by any suitable equipment well known to those skilled in the art.

To enable the operator to adjust the magnitude or amplitude of all of the pulse markers 21 simultaneously, I provide a variable attenuator circuit including a pair of fixed resistors and 2, connected in series, with a variable resistor ||3 connected between junction point l0 and ground, and with a fixed resistor ||4 connected between resistors ||I and ||2, and ground.

To complete the circuit of the pulse generator 2|, a conductor ||9 is connected between the `junction point 1| and a junction point H5, at

the output end of resistor ||2, to conduct the trigger pulse 20 to the junction point where it will effectively counteract the first pulse of each `of the multi-vibrator circuits, the trigger pulse being somewhat reduced in magnitude in so doing, but leaving the remaining marker pulses v21 unaffected. From junction point ||5, a connection is made by conductor H5 to the pulse amplifier 23 which may be an amplifier of any suitable conventional design which ampliiies or increases the magnitude of the trigger and marker pulses 20 and 21, and also reverses their polarity, as indicated by the diagrams 22 and 24 of the input and output voltages of the pulse amplier. From the pulse amplifier 23, the amplified pulse signals are transmitted to a bridge mixing pad |8 which likewise may be of a conventional design adapted to receive two separate input signals which may be mixed or combined in varying proportions, the signal appearing in one input circuit having no effect upon the other input circuit. From the bridge mixing pad I8 the combined output is transmitted to a gain set 28 which may be of any suitable type of attenuator control, adjustable to provide any desired amplitude of nal output signal.

The sound record produced by the apparatus illustrated in Figure 3 may be used to test the frequency response of a wide variety of different types of electronic equipment. One convenient arrangement for performing such tests is illustrated in Figure 4 wherein the record containing the repeated sweep frequency signal illustrated in Figure 2 is reproduced on a transcriber 20. The output of the transcriber is illustrated graphically by the signal 25, which is delivered to a piece of equipment |21 under test. The output signal |22 of the tested equipment 121 is fed to a cathode ray oscilloscope 123 wherein the wave pattern appears on the screen |24. The amplitude modulating envelope of the wave pattern |22 is, as has been stated, the frequency response curve of the tested equipment 12|. Thus it can be seen in the illustrated case that between 60 and 1,000 cycles, the frequency responsive equipment 12| is relatively fiat. Between l,000 and 3,000 cycles, the frequency response rises; between 3,000 and approximately 6,000 cycles the frequency drops, and from 6,000 to 10,000, it again rises to approximately normal level. By simple measurement of the pattern as it appears on the oscilloscope screen 124 using methods well known in the art, the actual decibel levels of the output signal 122 can be determined at any point along the frequency range.

Figures 6, '7 and S illustrate other non-linear frequency response patterns such as have been observed by the use of test records embodying my invention. Figure 6 is of particular interest as illustrating the type of output produced by imposing a relatively low frequency mechanical vibration on phonographic apparatus under test.

Figure 7 illustrates the appearance of an output curve having a relatively high response to low frequencies, being relatively :dat in the middle frequencies with a slight rise in response at the high frequencies. Figure S illustrates the output of equipment having a rising frequency response.

Since the duration of the synchronizing and y marking pulse is approximately 200 microseconds, its effect on audio circuits is substantially that of a square wave. Thus the synchronizing and marking pulses in addition to serving the purpose hereinbefcre described, may also .serve to check the transient response of the equipment under test.

Figures 9 and l0 illustrate the use of records embodying my invention for testing transient response. If the'cathode ray oscilloscope 124 used in connection with the test is so adjusted as to greatly expand the sweep rate, then pulse signals 21 will appear to be greatly magnified on the pattern as illustrated in Figure 9. The portion of the wave pattern immediately following the pulse 2l can then be examined to determine if the transient effect of the pulse is, or is not producing harmful or undesirable oscillation. In Figure 9 a pattern indicating relatively high transient distortion is indicated by the numeral 125. In Figure relatively low transient distortion is indicated by the numeral |26.

As was stated earlier herein, it is necessary for accurate measurement that the amplitude of the sweep frequency signal fed into the equipment under test be uniform throughout the frequency range. In order that this shall be true, it may in some instances be necessary to deliberately distortuthe amplitude modulation envelope of the signal as it is recorded in order to compensate for the normal alinear characteristics of the particular sound recording medium and transcription equipment used. For example, the

lil

response of an ordinary undulating groove sound record falls olf as the frequency increases. Thus if the signal delivered by a test record of this type embodying my invention is to be of uniform amplitude, it is necessary that the sweep frequency signal be recorded with an increasing amplitude as well as an increasing frequency. The technique for recording sound in a manner to produce faithful reproduction of amplitudes throughout a predetermined frequency range is sometimes referred to as orthacoustic recording, and such techniques being well known in the art, are not described in detail herein. Y

While I have found by experiment that a wid variety of recording media are suitable for `producing the sound record embodying my invention, I prefer to employ the conventional disc records of the constant radial velocity type such as may be played at '78 R. P. M. or in some cases, records designed to be played at 331/3 R.. P. M.

In carryingout my invention, I record the combined sweep frequency signals and pulse signals' by engraving a master disc from which pressings may be made in the conventional manner. In order that the pressings will produce a signal with a flat amplitude envelope throughout the range of frequencies recorded, I sometimes deliberately introduce alinear response elements into the gain set 26 illustrated in Figure 3 or elsewhere in the circuit so as to produce an amplitude envelope of the recorded signal which varies according to the standard curve established by the National Association of Broadcasters as that giving a substantially fiat frequency response for conventional disc transcriptions.

It should be noted that one important advan- Lage to be gained by testing audio equipment by the method described herein lies in the fact that, once recorded, the identity of the repeated notes remains uniform and the relative phase relation of the signal, marker pulses, and trigger pulses is Xed. This uniformity and unvarying time standard is due to the fact that the modulations in the sound track of the recording m-edium, are permanent in shape and fixed in their relative positions.

While the method and apparatus shown and described herein is fully capable of achieving the objects and providing the advantages hereinbefore stated, it is capable of considerable modification within the spirit of the invention. For this reason I do not mean to be limited to the specific forms, circuit arrangements, and constante her-ein described, but rather to the scope of the appended claims.

I claim:

l. In apparatus for producing a test record of the class described: a variable frequency signal generator adapted to produce signals of predetermined amplitude; frequency control means operatively associated with said generator and adapted to progressively vary said signal frequency at a predetermined rate over a predetermined frequency range; sweep circuit means oonnected to the output of said generator and controlled thereby, and connected to said frequency control means whereby to terminate and re-initiate the operation of said sweep circuit means at regular intervals whereby to produce a series of identical swept-frequency signals from said generator; and pulse generating means connected to said sweep circuit means to produce a pulse at the beginning of each of said signals, said pulse being of greater amplitude than that of said signal.

triggering pulse generating means responsive to said sweep circuit means and adapted to produce a triggering pulse at the beginning of each of said signals, said triggering pulse being of greater amplitude thanthat of said signal; and marker pulse generating means responsive to said trigger pulse means and adapted to superimpose on said signals at predetermined points therein, marker pulses of amplitude greater than that of said signal but less than that of said trigger pulses.

3. In apparatus for producing a test record of the class described: a variable frequency signal generator; frequency control means operatively associated with said generator Iand adapted to progressively Vary said signal frequency at a predetermined rate over a predetermined frequency range; sweep circuit means responsive to the output of said generator and operatively connected to said frequency control means whereby to terminate and re-initiate the operation of said sweep circuit at regular intervals whereby to produce a series of identical swept-frequency signals from said generator; trigger pulse generating means responsive to said sweep circuit means `and adapted to produce a pulse substantially simultaneously with each said re-nitiation of said signals; marker pulse generating means responsive to said trigger pulse means adapted to produce a plurality of marker pulses at predetermined times alter said trigger pulse, said marker pulses having less amplitude than said trigger pulses; means to amplify said pulses to amplitudes greater than that of said signal; and means to mix said pulses and signals whereby to produce a series of composite signals each having an initial pulse of predetermined amplitude, a wave pattern of constantly changing frequency and of amplitude less than that of said initial pulse, and a plurality of marker pulses occurring at predetermined frequency points in said wave pattern.

4. In apparatus for producing a test record of the class described: a variable frequency signal generator; frequency control means operatively associated with said generator and adapted to progressively vary said signal frequency over a predetermined frequency range; sweep circuit means connected to the output of said generator and controlled thereby, rand connected to said frequency control means whereby to terminate and re-initiate the operation of said sweep circuit at regular intervals whereby to produce a series of identical swept-frequency signals from said generator; and capacitor means in said sweep circuit means to vary the rate of change effected by said frequency control means whereby said signal frequency is varied exponentially.

5. In apparatus for producing a test record of the class described: a variable frequency signal generator; frequency control means operatively associated with said generator and adapted to progressively vary said signal frequency at a predetermined rate over a predetermined frequency range; yand sweep circuit means including an element responsive to a predetermined frequency connected between the output of said generator and said frequency control means to terminate and re-initiate the operation of said sweep circuit whenever said signal frequency reaches a predetermined value whereby to produce a series of identical swept frequency signals from said generator.

6. In apparatus for producing a test record of the class described: a tariable frequency signal generator; frequency control means operatively associated with said generator and adapted to progressively vary said signal frequency at a predetermined rate over a predetermined frequency range; and sweep circuit means including a reactive element selectively responsive to a predetermined frequency connected between the output of said generator and said frequency control means to terminate and re-initiate the operation of said sweep circuit whenever said signal frequency reaches a predetermined value whereby to produce a series of identical swept frequency signals from said generator.

WAYNE R. JOHNSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,661,751 Fletcher Mar. 6, 1928 1,792,528 Bleyer Feb. 17, 1931 1,961,367 Kuhn June 5, 1934 2,031,605 Jenkins et al Feb. 25, 1936 2,106,350 Hickman et al Jan. 25, 1938 2,110,090 Ligh et al. Mar. 1, 1938 2,142,591 Ross Jan. 3, 1939 2,144,844 Hickman Jan. 24, 1939 2,178,347 Piety Oct. 31, 1939 2,203,750 Sherman June 11, 1940 2,243,234 Von Duhn May 27, 1941 2,250,104 Morrison July 22, 1941 2,285,038 Loughlin June 2, 1942 2,296,919 Goldstine Sept. 29, 1942 2,304,633 Farnsworth Dec. 8, 1942 2,315,377 Pooh Mar. 30, 1943 2,373,275 Thomas Apr. 10, 1945 2,378,388 Begun June 19, 1945 2,403,982 Koenig, Jr July 16, 1946 2,403,986 Lacy July 16, 1946 2,419,569 Labin Apr.29, 1947 2,474,278 Ranger June 28, 1949 OTHER REFERENCES Bartholomew: Acoustics of Music, pages 21-26, Prentice-Hall, copyright 1942, copy in Division 69. 

